QuietStove.com

Monday, November 28, 2011

Gas Stoves: How Cold Can I Go?

UPDATE, 17 December 2014: I've almost completely re-written this article. I've tried to talk more about the overall process of planning for cold weather trips. I've also tried to talk about more than just the gas in a canister; I've added discussion of the practicalities of cold weather canister operation. One should not rely on the gas alone but also on one's own planning and techniques.

So, how cold can I go with my gas stove?

Good question. Do you want the short answer or the long answer?

The "Short" Answer
The short answer is that for the typical upright canister stove (that's the kind where the burner screws directly onto the canister), about 20°F/-7°C can be used as a rough planning number. For a longer answer, see the appendix. Understand though that the colder it gets, the more your stove will struggle even if it's warmer than my planning number. Your stove will not work just as well at 20°F/-7°C as it will at 75°F/24°C, at least not without planning and intervention on your part. You will need to know the basics of cold weather gas stove operation which are:

Start with a warm canister. Keep the canister in your sleeping bag with you before breakfast, put the canister under your clothing before supper, etc.

Keep the canister warm. Put some insulation under the canister (closed cell foam works well), protect the stove from wind, and put the canister in a bowl of (liquid!) water.

Heat the canister if needed. How can you tell if you need to heat the canister? Poor performance. If your flame is insipid in cold weather, you need to (carefully! slowly!) heat the canister. See my Stoves For Cold Weatherarticle in Seattle Backpackers Magazine for some ideas on how to heat the canister.

Now, I've described 20°F/-7°C as a rough planning number. What do I mean by that? Well, if you follow the basics of cold weather gas stove operation (select good gas, start with a warm canister, keep the canister warm, and heat the canister if needed) and use your head, it shouldn't be too difficult to keep an upright gas stove going in temperatures at or above the planning number. If you look at a weather forecast for the area and elevation in which you intend to use your stove, you should be able to get some idea of what temperatures you can expect. If the temperatures are well above 20°F/-7°C, you might plan to bring an upright canister stove. If the temperatures are forecast to be close to or below 20°F/-7°C, you might plan on bringing something else. For some alternatives, read the Alternatives for Colder Weather section of this article.

Note that 20°F/-7°C isn't carved in stone anywhere. This is just my fairly conservative rough planning number. Particularly with a fresh canister, you might be able to go colder. You may also get some cold weather performance increases at higher elevations where the boiling point of the fuel is lower. More on that in the appendix if you're interested. Note that this "planning number" is but one part of the overall planning process. There are many factors that should be considered when choosing gear for a particular trip, and this planning number is just one of them. My "planning number" is meant to be incorporated into one's overall planning process. It is not intended to trump the planning process.

Warning
I've said that the canister needs to be heated in cold weather in order to get good performance from a canister gas stove, but NEVER OVERHEAT A CANISTER. If the canister feels so hot to your bare hand that it's painful, that's too hot. Take IMMEDIATE action to reduce the heating of the canister. Overheating a canister can do a number of bad things including causing the canister to explode.

Crags and peaks, Sierra Nevada Mountains

Alternatives for Colder Weather
If you need to go colder than 20°F/-7°C, you have a couple of options. One is to switch from a canister gas stove to a liquid fueled stove, that is a stove fueled by white gasoline or kerosene (or similar). That's certainly a pretty tried and true approach.

But many people either don't feel comfortable with or don't want to hassle with liquid fueled stoves. In particular, many people are intimidated by having to prime a liquid fueled stove. Are there any options for canister gas in temperatures lower than my rough planning number of 20°F/-7°C? Well, if you want to use canister gas, you might want to bring a stove that can handle liquid feed gas, a remote canister stove with the canister inverted in other words. See my article, Stoves for Cold Weather II, for more information on liquid feed gas. With liquid feed gas stoves, one can use gas in colder temperatures, something on the order of 20 Fahrenheit/11 Celsius degrees colder than upright canister stoves – if you use your head and follow the basics of cold weather gas stove operation. My rough planning number for upright canister stoves is 20°F/-7°C and inverted canisters stoves have roughly a 20 Fahrenheit/11 Celsius degree advantage over upright stoves, therefore I use 0°F/-18°C as the rough planning number for inverted canister stoves.

Note that even with liquid feed gas, one still needs to have some pressure in the canister to drive the process. Therefore you must keep the canister "warm" relative to the outside environment. Placing the canister in water is a safe way to keep the canister warm. Water freezes at 32°F/0°C, so, as long as the water is liquid, your canister will be warmer than 32°F/0°C. Keeping the canister above 32°F/0°C should give you more than enough canister pressure to run your stove.

The planning numbers I suggest are guidelines for reducing risk; there are no guarantees in the back country. The planning numbers I suggest are temperatures in which you should be able to keep your fuel sufficiently warm to operate a canister gas stove. Keeping the canister warm requires intervention on your part. Note however that "stuff happens." If temperatures drop, if winds pick up, or if you are exhausted or injured, you may not be able to keep your fuel warm. You must be prepared to handle these and any other unexpected changes or emergencies that may occur. These planning numbers cannot be used alone. You must engage in proper planning, bring the proper equipment, and you must know and use the basics of cold weather canister operation.

Finally, the colder you go, the harder it is to keep the gas warm in a canister. If you're going out in really cold weather, ask yourself: "Can I really keep the canister warm?" In cold weather, an abundance of caution is the best advice I can give you.

HJ

Appendix – The "Long" Answer

So, what am I basing my planning numbers on? Well, let's talk a little bit, at a high level, about the chemistry of canister gas. If this gets too boring or confusing or if you disagree or you just plain don't like what I've written, I commend to you a much simpler approach in Cold Weather Tips for Gas Stoves.
Now for the "long" answer: Canisters for backpacking typically contain propane mixed with either isobutane or "plain" butane (n-butane). The boiling points (vaporization points) at sea level of each of these gases are as follows:

When you blend the fuels together, your boiling point will be somewhere in between the boiling point of the various fuels. Calculating the exact boiling point is not trivial. Not only that, but the proportions of the fuel mix will change over time with an upright canister stove. Why will the mix change over time? If the canister is used in the normal, upright position, the propane will boil off at a faster rate than either n-butane or isobutane because if its higher vapor pressure. Since the propane is boiling off faster, there will be less and less propane in the mix over time. As the percentage that is propane declines, the boiling point rises.

OK, so why all this blather about boiling points? Well, the boiling point is the temperature at which your fuel will turn to vapor, i.e. change from liquid to gas. Canister gas stoves require gas to operate, but that's a liquid inside your canister. Just shake the canister if you don't believe me. That's a liquid in there. The fuel inside the canister is under such high pressure that the gas liquefies (turns into a liquid). In order to run a canister gas stove, that fuel has to vaporize. In order to vaporize, the fuel must boil. If the fuel temperature is lower than the boiling point, it won't vaporize, and your stove won't run. For canister gas stoves in cold weather, we want the lowest boiling point possible so that the fuel will turn to gas so that we can run our stove. We must then keep our canister warmer than the boiling point of the fuel.

Well, OK, if the propane is burning off and the boiling point is constantly changing, how the heck can I figure out how warm I have to keep the canister!? Well, toward the end of a canister, all of your propane will have burned off, and you will just have the fuel with the highest boiling point remaining (either isobutane or n-butane). Therefore the boiling point of your fuel will never be higher than the boiling point of the fuel that remains at the end of a canister. All we have to do now is keep the fuel temperature is above the boiling point of the worst cold weather performing fuel. Isobutane boils at 11°F/-12°C; n-butane boils at 31°F/-0.5°C. Isobutane therefore is a much better fuel in cold weather. This is why I stress that one should avoid n-butane and only purchase fuels containing isobutane and propane for cold weather use.

I've just described in the preceding paragraph how the propane burns off more quickly when the canister is used in upright mode. If you read the "Alternatives for Colder Weather" section, I talk about using a stove that can handle the canister in inverted (upside down) mode. One of the advantages of using a canister in inverted mode is that your fuel mixture doesn't change much over time. Since you're pulling liquid off the bottom instead of gas off the top, it doesn't matter which fuel boils at what rate so long as there's enough boiling to produce sufficient canister pressure. Since you're not losing your propane (your best cold weather fuel), your boiling point stays low. Remember, you want the lowest boiling point fuel for cold weather.

Now there's another matter: Canister pressure. Just being above the boiling point isn't necessarily enough. If our fuel boils at 31°F/-0.5°C, and the fuel temperature is 32°F/0°C, then it's not going to boil very vigorously, and we're really not going to have enough pressure to run a stove properly. To illustrate this, think of the "chattering" lid on a pot of boiling water. If the water is just barely boiling, the lid is quiet. But if you turn up the heat, the water will start boiling vigorously, producing enough steam to literally lift the lid for a moment. The lifting (as the pressure builds up) and falling (after the pressure is released) is what causes the noise of a "chattering" lid on a pot of boiling water. In order to run a stove, you want the fuel temperature to be high enough to produce a vigorous boil. How high above the boiling point does the fuel temperature have to be to have enough pressure to properly run a stove? Well, I suppose it depends on your definition of "properly" (as well as some external factors like atmospheric pressure and temperature), but the bottom line is that if you turn on the stove and the flame isn't strong enough to cook with, then the fuel is most likely too cold and you may have to warm the canister. If you want numbers, if the fuel temperature is 20°F/7°C degrees above the boiling point of the worst performing fuel, then generally you should have good canister pressure. The performance of your stove, however, is the bottom line as to whether or not your fuel is warm enough. Caution: If you heat the canister, and performance doesn't improve, think. Could there be some other problem, for example, a blockage in the jet or an empty canister? If you keep adding heat and performance doesn't improve, there's something else going on. Never heat the canister to the degree that the canister feels painfully hot.

And there's yet another issue: Canisters get colder as you operate the stove, through the process of evaporative cooling. Canisters cool from within and get colder than their surroundings. Take a look at the below photo.

Ice forming on the lower portion of a canister in cold but not freezing weather

That's ice forming on the sides of the canister even though the temperature is not below freezing. Why? Because as I use the gas, evaporative cooling occurs, and the canister gets colder than its surroundings. So starting with a canister that is warm enough in the beginning is no guarantee that the canister will stay warm enough throughout the time that you are cooking. This is why I suggest heating the canister if needed. Heating the canister counteracts the effects of internal canister cooling as well as cooling due to heat lost to the environment. Here's another advantage to using stoves with the canister inverted: the canister doesn't cool from within to the same degree as a canister does when you use it upright. Thus, there are two advantages to using the canister upside down (if your stove will support it!):
1. Your propane won't be leaving the mix.
2. Your canister won't get as cold.
More propane + warmer canister = better pressure. Inverted canister stoves are definitely superior for cold weather.

Now, another complication, but this one generally works in our favor: As you ascend, the higher you go, the lower the atmospheric pressure. As the atmospheric pressure drops, so does the boiling point of your fuel. Generally, the drop is about 1.0 degree Fahrenheit for every thousand feet in elevation gained. In metric units, that's about 0.5 degrees Celsius colder for every 300 meters in elevation gained. Note the use of the word "about." The actual values are higher, but I'm going to be conservative here. I'll say more about why in a moment. Take a look at the below chart.

A chart showing:The relationship between boiling point and elevation for isobutane, navy blue line, Temperature vs. elevation for an internal canister pressure of 5 psi gauge (psig) for isobutane, red line.Temperature vs. elevation for an internal canister pressure of 10 psi gauge (psig) for isobutane, gold line.The relationship between the freezing point of water and elevation, green line.Values are illustrative not exact.

The navy blue line shows how the boiling point gets lower as elevation increases. Now, of course it's atmospheric pressure that's the real issue, but since atmospheric pressure declines relative to elevation at a fairly steady rate over the interval shown here, I show elevation. In reality atmospheric pressure varies not only with elevation but also with conditions, and the change relative to elevation is not really linear (but it's close). The chart is a simplification of what's really going on. But recall that we're just after a rough planning number, a planning number that will be incorporated into our overall trip planning process, not a number that will override all else, so this chart is sufficient for our purposes.

OK, now why do I only use 1 degree Fahrenheit per 1000 feet in elevation gain (about 0.5 C per 300m) even though the actual change in boiling point is (slightly) greater? Well, let's look at the red and gold lines on the above chart. The red and gold lines illustrate what we're really after: Canister pressure. Notice that the red and gold lines are not parallel to the navy blue line. As it happens, the pressure inside a canister is lower in cold even if we're the same number of degrees above the boiling point. In colder temperatures, the fuel temperature must be higher above the boiling point to produce the same internal canister pressure. Notice how the red lines and gold lines get progressively farther away from the navy blue line. The red and gold lines represent a constant pressure (5 psig and 10 psig, respectively). In order to maintain a constant pressure, the red and gold lines have to get progressively farther away from the navy blue line as temperatures get colder. Note that these lines are not exact; in fact I've exaggerated them slightly for illustrative purposes. Also, the actual functions are not linear in progression (but they're close). We're not planning a Mars mission here. We just need a rough planning number. Approximating the true, non-linear progression with a linear one will get us in the ballpark.

An MSR Reactor operating at over 23,000 feet/7000 meters elevation on Muztagh Ata (24636'/7509m) in far western China. Overnight lows approached -40°F (which is also -40°C; the scales are equal at -40°). One person died on the mountain during this expedition. It was cold. Note that the canister is labeled "Isobutane" on the side.
Photo credit: Reuben Brimfield

Another note: You can't out climb low canister pressure. Some people have jumped to the conclusion that I'm saying "just keep climbing to the elevation where the boiling point is so low that your stove will start working." Uh, no. Temperatures fall faster than boiling points. In other words, as you climb, it's going to get colder faster than the elevation change will improve your stove's performance. The proper thing to do is to get a good understanding of the temperatures you'll be facing on a given outing and be prepared for them. Don't try to climb to improve your stove's performance.

Now, look at the green line. This represents the freezing point of water. Note that the line is flat. The freezing point of water does not vary with elevation the way that the boiling point of our fuel does. Any liquid water will generally be warmer than the temperature that the green line represents. Yes, the presence of certain minerals can change the freezing point of water, but for the purposes of our rough planning number, we will assume that any water that is liquid will be warmer than the green line. Note that the green line is almost always above the gold line. In other words, if we put a canister in water, you should have good canister pressure, so long as the water remains liquid. I'm assuming here that you followed the other basics of cold weather canister operation as well. This is the beauty of using water as a method of heating the canister. By the mere fact that the water is liquid, we generally will have enough pressure with which to cook. However, if you're not getting the performance you need, you should be prepared to take action. One action might be to take some water out of the pot that you're heating and put it into the bowl that the canister sits in. Don't get it too hot! If you add just a touch of warmer water (slowly!) to the water that is already in the bowl, you will bring the water temperature up a bit and increase your stove's performance. There are other ways to heat the canister, but liquid water is pretty safe so long as the canister never feels hot to the point of pain to the (unfrozen) bare hand.

Of course you can modify the stove to divert heat to the canister from the flame directly instead of using water, but there are risks in doing so. Be very careful. Overheated canisters can and do explode. A canister explosion is a potentially life threatening event. ALWAYS monitor the canister temperature with your (unfrozen) hand. If the canister feels hot to the touch, turn it down immediately.

Now you may be thinking, "if I can heat the canister, what's up with these planning numbers you keep talking about? Shouldn't I be able to go as cold as I want so long as I keep the canister warm?" Well, yes you can. My planning numbers are an attempt to be conservative and thereby reduce risk (reduce, not eliminate). The colder it gets, the harder it is to keep the canister warm. If you're going out in really cold weather with a gas stove, think. Can you really keep your canister warm in those temperatures? What happens if the temperature is colder than you expected? Can you compensate? In really cold weather, your life may depend on your stove functioning. Take no short cuts.

Summary:
1. If you use a canister right side up, the best cold weather fuel, propane, boils off at a faster rate, so you must be able to rely on the other components of your fuel. Therefore choose isobutane and avoid n-butane.
2. If you use a canister upside down (inverted), the propane stays in the mix and your fuel has better cold weather performance. Most stoves cannot handle inverted operation. Do your homework before trying this.
3. If used upright, canisters experience significant cooling from within. Therefore, it is the fuel temperature which matters, not the ambient temperature. Your fuel temperature will be usually be colder than the surroundings after operating the stove for a while.
4. If you use a canister upside down, the canister will not experience cooling (well, at least not to the degree that it does in upright operation).
5. In order to have enough pressure to properly operate a stove, your fuel temperature must be warmer than the vaporization point (boiling point) of the fuel. Generally, about 20°F/11°C degrees above the vaporization point will give you good operating pressure, but the actual performance of the stove is the bottom line. Poor performance probably means that your fuel needs more heat. Therefore you must be able to heat the canister. Water is typically a safe way to heat the canister.
6. NEVER heat a canister to the degree that it is painfully hot to the touch of an (unfrozen) bare hand.
7. The higher you go, the colder the weather your gas stove will operate in, but the colder it gets, the harder it is to keep the canister warm, irrespective of elevation. As you climb, temperatures fall faster than the performance of your stove increases. You cannot out climb cold.
8. If you heat the canister, you are not as constrained by the ambient temperature. However common sense still applies here. Can you realistically keep the canister warm enough in the temperatures expected? What happens if the weather is colder than expected? What happens if a storm moves in?
9. You must know and use the basics of cold weather canister operation (select good fuel, start with a warm canister, keep the canister warm, and heat the canister if needed). You must also be prepared for emergencies and the unexpected.

4 comments:

Canister gas is really liquid fuel, but gas stoves lack vaporizers, except for those intended for liquid feed. The heat of vaporization comes from the atmosphere and must occur at a rate sufficient to keep the liquified gas temperature up. There are all sorts of trade offs, but I'm just not a canister gas fan. I prefer liquid fuel in most cases.